During the maturation of spirits such as whisky, brandy etc, a certain amount of spirit, colloquially called “the angels' share” in the industry, is lost from the cask by the evaporation of the spirit through the material of the cask. It is estimated that approximately two % per year of the spirit within the cask is lost through the evaporation of the angels' share. Although the evaporation of the spirit from the cask during the maturation process represents an economic loss to the distiller, it is tolerated because attempts to reduce the amount of fluid evaporating from the cask have been detrimental to the quality of the remaining spirit produced.
Apart from the economic loss represented by the evaporation of the spirit from the cask, there are other disadvantages. Distilleries and other sites where spirits are matured are commonly affected by “blackening” of the surfaces of nearby buildings and other structures. This is caused by the accelerated growth of a naturally occurring fungus Baudoinia compniacensis, which thrives in environments rich in ethanol vapour.